The invention relates to a method for measuring a fluid, wherein the consistency, flow and/or gas content of a flowing fluid is measured by means of electromagnetic radiation.
The invention also relates to an arrangement for measuring a fluid, comprising units for measuring the consistency, flow and/or gas content in order to measure a fluid that flows in a process pipe by means of electromagnetic radiation.
Flow measuring equipments are generally used to measure the flow of a fluid in a pipe. There are various types of prior art measuring equipments, which are typically based on such physical phenomena as the Coriolis force, pressure difference and voltage induced by movement of a conductive material in a magnetic field. Further, ultrasound technology has been used to implement measuring equipments based on correlation and the Doppler effect. Microwave technology has also been used to provide flow measuring equipments based on the Doppler effect. There are also arrangements utilizing the correlation of microwaves by means of a metallic process pipe, disclosed for example in U.S. Pat. No. 4,423,623, U.S. Pat. No. 4,888,547 and WO 94/17373. In the arrangements disclosed in U.S. Pat. Nos. 4,423,623 and 4,888,547, the process pipe is used as a waveguide, and variations in the cut-off frequency of the waveguide serve as correlating signals. The arrangement of WO 94/17373 employs the correlation of signals on the same frequency or at least the same frequency band after the signals have propagated through the flowing material.
The manufacture of paper of good quality requires accurate measurement and adjustment of the water or solids content of the papermaking pulp, in other words the pulp consistency. If the consistency of the pulp is too low or too high, the paper web will not remain homogenous and the quality of the finished paper will not be as good as possible.
At present, the gas content and especially the air content of a fluid or a liquid substance are measured mainly by means of methods and devices based on the ultrasound and the measurement of density. In the ultrasound measurement, the ultrasound is transmitted through the fluid to be measured and the attenuation of the ultrasound is measured. The attenuation of the ultrasound is a function of the gas content of the fluid: the higher the gas content the greater the attenuation of the ultrasound. In the paper industry, the gas content of papermaking pulp with a consistency below 2% is typically measured by means of the ultrasound. The quality of the final product, i.e. paper, depends on the quality of the liquid pulp, which, in turn, is partly dependent on the gas content thereof.
The purpose of the invention is to provide a method and an apparatus, implementing the method with which the aforementioned problems can be solved. This is achieved with a method of the type described in the introduction, which is characterized in that at least two of said measurements are carried out substantially from the same measurement point in the fluid by utilizing one integrated measuring equipment comprising units for measuring at least said two properties for the transmission and reception of electromagnetic radiation.
The arrangement according to the invention is characterized in that at least two measuring units, intended for the transmission and reception of electromagnetic radiation, from the units for measuring the consistency, flow and/or gas content are integrated into one measuring equipment, which is attached to the pipe and arranged to perform at least two of said measurements substantially from the same measurement point in the fluid.
The method and the arrangement according to the invention provide several advantages. It is possible to measure with one measuring equipment several properties of a fluid, such as a suspension or slurry, from one measurement point in the process, which reduces the costs of mounting the measuring equipment. Further, one measuring equipment requires less cabling than two or more separate measuring equipments. Particularly air mixed in the papermaking pulp also affects the measurement of production that is based on the consistency and the flow, since with a high air content the process pipe is not full as it is assumed to be in the measurement of the production.